The line switches of one hundred lines are all a.s.sociated to form a single unit of apparatus, which, besides the individual line switches, includes certain other apparatus common to those lines. Such a group of one hundred line switches and a.s.sociated common apparatus is called a _line-switch unit_, or frequently, a _Keith unit_. Confusion is likely to arise in the mind of the reader between the individual line switch and the line-switch unit, and to avoid this we will refer to the piece of apparatus individual to the line as the line switch, and to the complete unit formed of one hundred of these devices as a line-switch unit.
_Line and Trunk Contacts._ Each line switch has its own bank of contacts arranged in the arc of a circle, and in this same arc are also placed the contacts of each of the ten individual trunks which it is possible for that line to appropriate. The contacts individual to the subscriber"s line in the line switch are all multipled together, the arrangement being such that if a wedge or plunger is inserted at any point, the line contacts will be squeezed out of their normal position so as to engage the contacts of the trunk corresponding to the particular position in the arc at which the wedge or plunger is inserted. A small plunger individual to each line is so arranged that it may be thrust in between the contact springs in the line-switch bank in such manner as to connect any one of the trunks with the line terminals represented in that row, the particular trunk so connected depending on the portion of the arc toward which the plunger is pointed at the time it is thrust in the contacts.
These banks of lines and trunk contacts are horizontally arranged, and piled in vertical columns of twenty-five line switches each. The ten trunk contacts are multipled vertically through the line-switch banks, so that the same ten trunks are available to each of the twenty-five lines. We thus have, in effect, an old style, Western Union, cross-bar switchboard, the line contacts being represented in horizontal rows and the trunk contacts in vertical rows, the connection between any line and any trunk being completed by inserting a plunger at the point of intersection of the horizontal and the vertical rows corresponding to that line and trunk.
_Trunk Selection._ The plungers by which the lines and trunks are connected are, as has been said, individual to the line, and all of the twenty-five plungers in a vertical row are mounted in such manner as to be normally held in the same vertical plane, and this vertical plane is made to oscillate back and forth by an oscillating shaft so as always _to point the plungers toward a vertical row of trunk contacts that represent a trunk that is not in use at the time_. The to-and-fro movement of this oscillating shaft, called the _master bar_, is controlled by a master switch and the function of this master switch is always to keep the plungers pointed toward the row of contacts of an idle trunk. The thrusting movement of the individual plungers into the contact bank is controlled by magnets individual to the line and under control of the subscriber in initiating a call. As soon as the plunger of a line has been thus thrust into the contact bank so as to connect the terminals of that line with a given trunk, the plunger is no longer controlled by the master bar and remains stationary. The master bar then at once moves all of the other plungers that are not in use so that they will point to the terminals of another trunk that is not in use. The plungers of all the line switches in a group of twenty-five are, therefore, subject to the oscillating movements of the master bar when the line is not connected to a first selector trunk. As soon as a call is originated on a line, the corresponding plunger is forced into the bank and is held stationary in maintaining the connection to a first selector trunk, and all of the other plungers not so engaged, move on so as to be ready to engage another idle trunk.
_Trunk Ratio._ The a.s.signment of ten trunks to twenty-five lines would be a greater ratio of trunks than ordinary traffic conditions require.
This ratio of trunks to lines is, however, readily varied by multipling the trunk contacts of several twenty-five line groups together. Thus, ten trunks may be made available to one hundred subscribers" lines by multipling the trunks of four twenty-five line switch groups together.
In this case the four master bars corresponding to the four groups of twenty-five line switches are all mechanically connected together so as to move in unison under the control of a single master switch. If more than ten and less than twenty-one trunks are a.s.signed to one hundred lines, then each set of ten trunks is multipled to the trunk contacts of fifty line switches, the two master bars of these switches being connected together and controlled by a common master switch.
_Structure of Line Switch._ The details of the parts of a line switch that are individual to the line are shown in Fig. 385, the line and trunk contact bank being shown in the lower portion of this figure and also in a separate view in the detached figure at the right. A detailed group of several such line switches with the oscillating master bar is shown in Fig. 386. This figure shows quite clearly the relative arrangement of the line and trunk contact banks, the plungers for each bank, and the master bar.
[Ill.u.s.tration: Fig. 385. Line Switch]
In practice, four groups of twenty-five line switches each are mounted on a single framework and the group of one hundred line switches, together with certain other portions of the apparatus that will be referred to later, form a line-switch unit. A front view of such a unit is shown in Fig. 387. In order to give access to all portions of the wiring and apparatus, the framework supporting each column of fifty line switches is hinged so as to open up the interior of the device as a whole. A line-switch unit thus opened out is shown in Fig. 388.
[Ill.u.s.tration: Fig. 386. Portion of Line-Switch Unit]
_Circuit Operation._ The mode of operation of the line switch may be best understood in connection with Fig. 389, which shows in a schematic way the parts of a line switch that are individual to a subscriber"s line, and also those that are common to a group of fifty or one hundred lines. Those portions of Fig. 389 which are individual to the line are shown below the dotted line extending across the page. The task of understanding the line switch will be made somewhat easier if Figs. 385 and 389 are considered together. The individual parts of the line switch are shown in the same relation to each other in these two figures with the exception that the bank of line and trunk springs in the lower right-hand corner of Fig. 389 have been turned around edgewise so as to make an understanding of their circuit connections possible.
[Ill.u.s.tration: Fig. 387. Line-Switch Unit]
[Ill.u.s.tration: Fig. 388. Line-Switch Unit]
[Ill.u.s.tration: Fig. 389. Circuits of Line-Switch Unit]
The vertical and rotary sides of the subscriber"s line are shown entering at the lower left-hand corner of this figure, and they pa.s.s to the springs of the contact bank. Immediately adjacent to these springs are the trunk contacts from which the vertical and the rotary limbs of the first selector trunk proceed. The plunger is indicated at _1_, it being in the form of a wheel of insulating material. It is carried on the rod _2_ pivoted on a lever _3_, which, in turn, is pivoted at _4_ in a stationary portion of the framework. A spring _5_, secured to the underside of the lever _3_ and projecting to the left beyond the pivot _4_ of this lever, serves always to press the right-hand portion of the lever _3_ forward in such direction as to tend to thrust it into the contact bank. The plunger is normally held out of the contact bank by means of the latch _6_ carried on the armature _7_ of the trip magnet.
When the trip magnet is energized it pulls the armature _7_ to the left and thus releases the plunger and allows it to enter the contact bank.
[Ill.u.s.tration: POWER SWITCHBOARD FOR MEDIUM-SIZED OFFICE Mercury Arc Rectifier Panel and Transformer at Right.]
The master bar is shown at _8_, and a feather on this bar engages a notch in the segment attached to the rear end of the plunger rod _2_.
This master bar is common to all of the plunger rods and by its oscillatory movement, under the influence of the master switch, it always keeps all of the idle plunger bars pointed toward the contacts of an idle trunk. As soon, however, as the trip magnet is operated to cause the insertion of a plunger into the contact bank, the feather on the master bar is disengaged by the notch in the segment of the plunger rod, and the plunger rod is, therefore, no longer subject to the oscillating movement of the master bar.
When the release magnet is energized, it attracts its armature _9_ and this lifts the armature _7_ of the trip magnet so that the latch _6_ rides on top of the left-hand end of the lever _3_. Then, when the release magnet is de-energized, the spring _5_, which was put under tension by the latch, moves the entire structure of levers back to its normal position, withdrawing the plunger from the bank of contacts. The notch on the edge of the segment of the plunger rod, when thus released, will probably not strike the feather on the master bar, and the plunger rod will thus not come under the control of the master bar until the master bar has moved, in its oscillation, so that the feather registers with the notch, after which this bar will move with all the others.
If, while the plunger is waiting to be picked up by the master bar, the same subscriber should call again, his line will be connected with the same trunk as before. There is no danger in this, however, that the trunk will be found busy, because the master bar will not have occupied a position which would make it possible for any of the lines to appropriate this trunk during the intervening time.
_Master Switch._ a.s.sociated with each master bar there is a master switch which determines the position in which the master bar shall stop in order that the idle plungers may be pointed always to the contacts of an idle trunk. The arm _10_ of this switch is attached to the master bar and oscillates with it and serves to connect the segment _11_ successively with the contacts _12_, which are connected respectively to the third, or release wire of each first selector trunk. In the figure the arm _10_ is shown resting on the sixth contact of the switch and this sixth contact is connected to a spring _13_ in the line-switch contact bank that has not yet been referred to. As soon as the plunger is inserted into the contact bank, the spring _14_ will be pressed into engagement with the spring _13_, and this spring _14_ is connected with the live side of the battery through the release magnet winding.
The contact strip _11_ on the master switch is thus connected through the release magnet to the battery and from this current flows through the left-hand winding of the master-switch relay. This energizes this relay and causes the closure of the circuit of the locking magnet which magnet unlocks the master bar to permit its further rotation. The unlocking of the master bar brings the spring _15_ into engagement with _16_ and thus energizes the master magnet, the armature of which vibrates back and forth after the manner of an electric-bell armature, and steps the wheel _17_ around. The wheel _17_ is mechanically connected to the master bar so that each complete revolution of the wheel will cause one complete oscillation of the master bar. The master bar will thus be moved so as to cause all the idle plungers to sweep through an arc and this movement will stop as soon as the master-switch arm _10_ connects the arc _11_ with one of the contacts _12_ that is not connected to the live side of the battery through the springs _13_ and _14_ of some other line switch. It is by this means that the plungers of the line switches are always kept pointing at the contacts of an idle trunk. The way in which this feature has been worked out must demand admiration and accounts for the marvelous quickness of this line switch.
The fact that the plungers are pointed in the right direction before the time comes for their use, leaves only the simple thrusting motion of the plunger to accomplish the desired connection immediately upon the initiation of a call by the subscriber.
_Locking Segment._ It will be understood that the locking segment _18_ and the master-switch contact finger _10_ are both rigidly connected with the master bar _8_ and move with it, the locking segment _18_ serving always to determine accurately the angular position at which the master bar and the master-switch arm are brought to rest.
_Bridge Cut-Off._ One important feature of automatic switching, particularly as exemplified in the system of the Automatic Electric Company, is the disconnection, after its use, of each operating magnet of each piece of apparatus involved in making a connection. Since these operating magnets are always bridged across the line at the time of their operation and then cut off after they have performed their function, this feature may be referred to as the _bridge cut-off_.
_Guarding Functions._ Still another feature of importance is the means for guarding a line or a piece of apparatus that has already been appropriated or made busy, so that it will not be appropriated or connected with for use in some other connection. For this latter purpose contacts and wires are a.s.sociated with each piece of apparatus, which are multipled to similar contacts on other pieces of apparatus in much the same way and for a similar purpose that the test thimbles in a multiple switchboard are multipled together. Such wires and contacts in the Automatic Electric Company"s apparatus are called _private wires_ and _contacts_.
The bridge cut-off and guarding functions are provided for in the line switch by a bridge cut-off relay shown in Fig. 389 and also in Fig. 385, it being the upper one of the individual line relays in each of those figures. This bridge cut-off relay is operated as soon as the plunger of the line is thrust into the bank; the contacts _19_ and _20_, closed by the plunger, serving to complete the circuit of this relay. To make clear the bridge cut-off feature it will be noted that the trip magnet of a line switch is connected in a circuit traced from the rotary side of the line through the contacts _21_ and _22_ of the bridge cut-off relay, thence through the coil of the trip magnet to the common wire leading to the spring _23_ of the master-bar locking device and thence to the live side of the battery. Obviously, therefore, as soon as the bridge cut-off relay operates, the trip magnet becomes inoperative and can cause no further action of the line switch because its circuit is broken between the springs _21_ and _22_.
The private or guarding feature is taken care of by the action of the plunger in closing contacts _19_ and _20_, since the private wire leading to the bridge cut-off relay is, as has already been stated, connected to ground when these contacts are closed. This private wire leads off and is multipled to the private contacts on all the connectors that have the ability to reach this line, and the fact that this wire is grounded by the line switch as soon as it becomes busy, establishes such conditions at all of the connectors that they will refuse to connect with this line as long as it is busy, in a way that will be pointed out later on.
_Relation of Line Switch and Connectors._ The vertical and rotary wires of the subscriber"s line are shown leading off to the connector banks at the left-hand side of Fig. 389, and one side of this connection pa.s.ses through the contacts _24_ and _25_ of the bridge cut-off relay on the line switch. It is through this path that a connection from some other line through a connector to this line is established and it is seen that this path is held open until the bridge cut-off relay of the line switch is operated. For such a connection to this line the bridge cut-off relay of the line switch is operated over the private wire leading from the connector, and the operation of the bridge cut-off relay at this time serves to render inoperative the line switch, so that it will not perform its usual functions should the called subscriber start to make a call after his line had been seized.
_Summary of Line-Switch Operation._ To summarize the operation of a line switch when a call is originated on its line, the first movement of the calling subscriber"s dial will ground the rotary side of the line and operate the trip magnet. This will cause the plunger to be inserted into the bank, and thus extend the line to the first selector trunk through the closing of the right-hand set of springs shown in the lower right-hand corner of Fig. 389. The insertion of the plunger will also connect the battery through the left-hand winding of the master-switch relay and, by the sequence of operations which follows, cause the master bar to move all of the idle plungers so as to again point them to an idle trunk. The closure of contacts _19_ and _20_ by the plunger causes the operation of the bridge cut-off relay which opens the circuit of the trip magnet, rendering it inoperative; and also establishes ground potential on all the private wire contacts of that line in the banks of the connectors, so as to guard the line and its a.s.sociated apparatus against intrusion by others. The line is cut through, therefore, to a first selector and all of the line-switch apparatus is completely cut off from the talking circuit.
It must be remembered that all of the actions of the line switch, which it has taken so long to describe, occur practically instantaneously and as a result of the first part of the first movement of the subscriber"s dial. The line switch has done its work and "gone out of business"
before the selective impulses of the first digit begin to take place.
=Selecting Switches.= The first selector is now in control of the calling subscriber. The circuits and elements of the first selector switch are shown in Fig. 390. The general mechanical structure of the first selectors, second selectors, and connectors, is the same and may be referred to briefly here. Fig. 391 shows a rear view of a first selector; Fig. 392, a side view of a second selector; and Fig. 393, a front view of a connector. The arrangement of the vertical and rotary magnets, of the selector shafts, and of the contact banks are identical in all three of these pieces of apparatus and all these switches work on the "up-and-around principle" referred to in connection with Fig. 380.
It is thought that with the general structure shown in Figs. 391, 392, and 393 in mind, the actual operation may be understood much more readily from Fig. 390.
Four magnets--the vertical, the rotary, the private, and the release--produce the switching movements of the machine. These magnets are controlled by various combinations brought upon the circuits by three relays--the vertical, the rotary, and the back release. The fourth relay shown, called the _off-normal_, is purely for signaling purposes, as will be described.
_Side Switch._ Another important element of the selecting switches is the so-called side switch which might better be called a pilot switch--but we are not responsible for its name. This side switch has for its function the changing of the control of the subscriber"s line to successive portions of the selector mechanism, rendering inoperative those portions that have already performed their functions and that, therefore, are no longer needed. This switch may be seen best in Fig.
392 just above the upper bank of contacts. It is shown in Fig. 390 greatly distorted mechanically so as to better ill.u.s.trate its electrical functions.
[Ill.u.s.tration: Fig. 390. Circuits of First Selector]
The contact levers _1_, _2_, _3_, and _4_ of the side switch are carried upon the arm _5_ which is pivoted at _6_. All of these contact levers, therefore, move about _6_ as an axis. The side switch has three positions and it is shown, in Fig. 390, in the first one of these. When the private magnet armature is attracted and released once, the escapement carried by it permits the spring _7_ to move the arm _5_ so as to bring the wipers of the side switch into its second position; the second pulling up and release of the private magnet armature will cause the movement of the side switch wipers into the third position. It is to be noted that the escapement which releases the side switch arm may be moved either by the private or by the rotary magnet, since the armature of the latter has a finger which engages the private magnet armature.
[Ill.u.s.tration: Fig. 391. Rear View of First Selector]
_Functions of Side Switch._ The functions of the side switch may be briefly outlined in connection with the first selector, as an example.
In the first position it extends the control of the subscriber"s signal transmitter through the first selector trunk and line relays to the vertical and private magnets so that these magnets will be responsive to the selecting impulses corresponding to the first digit. In its second position it brings about such a condition of affairs that the rotary magnet will be brought into play and automatically move the wipers over the bank contacts in search of an idle trunk. In its third position, both the vertical and rotary relays are cut off and the line is cut straight through to the second selector trunk, and only those parts of the first selector apparatus are left in an operative state which have to do with the private or guarding circuits and with the release.
Similar functions are performed by the side switch in connection with the other selecting switches.
[Ill.u.s.tration: Fig. 392. Side View of Second Selector]
_Release Mechanism._ Another one of the features of the switch that needs to be considered before a detailed understanding of its operation may be had, is the mechanical relation of the holding and the release dog. This dog is shown at _8_ and, in the language of the art, is called the _double dog_. As will be seen, it has two retaining fingers, one adapted to engage the vertical ratchet and the other, the rotary ratchet on the selector shaft. This double dog is pivoted at _9_ and is interlinked in a peculiar way with the armature of the vertical magnet, the armature of the release magnet, and the arm of the side switch. The function of this double dog is to hold the shaft in whatever vertical position it is moved by the vertical magnet and then, when the rotary magnet begins to operate, to hold the shaft in its proper angular position. It will be noted that the fixed dog _10_ is ineffective when the shaft is in its normal angular position. But as soon as the shaft is rotated, this fixed dog _10_ becomes the real holding pawl so far as the vertical movement is concerned. The double dog _8_ is normally held out of engagement with the vertical and the rotary ratchets by virtue of the link connection, shown at _11_, between the release magnet armature and the rear end of the double dog. On the previous release of the switch the attraction of the release magnet armature permitted the link _11_ to hook over the end of the dog _8_ and thus, on its return movement, to pull this dog out of engagement with its ratchets. This movement also resulted in pushing on the link _12_ which is pivoted to the side switch arm _5_, and thus the return movement of the release magnet is made to restore the side switch to its normal position. In order that the double dog may be made effective when it is required, and in order that the side switch may be free to move under the influence of the private magnet, the double dog is released from its connection with the release magnet armature by the first movement of the vertical magnet in a manner which is clear from the drawing.
=First Selector Operation.= In discussing the details of operation of the various selectors it will be found convenient to divide the discussion according to the position of the side switch. This will bring about a logical arrangement because it is really the side switch which determines by its position the sequence of operation.
[Ill.u.s.tration: Fig. 393. Front View of Connector]
_First Position of Side Switch._ This is the position shown in Fig. 390, and is the normal position. The vertical and the rotary lines extending from the calling subscriber are continued by the levers _1_ and _2_ of the side switch through the vertical and the rotary relay coils, respectively, to the live side of battery. The lever _4_ of the side switch in this position connects to ground the circuit leading from the line switch through the release trunk, and the winding of the off-normal relay. This winding is thus put in series with the release magnet of the line switch, but on account of high resistance of the off-normal relay no operation of the release magnet is caused. This will, however, permit such current to flow through the release circuit as will energize the sensitive off-normal relay and cause it to attract its armature and light the off-normal lamp. If this lamp remains lighted more than a brief period of time, it will attract notice and will indicate that the corresponding selector has been appropriated by a line switch and that for some reason the selector has gone no further. This lamp, therefore, is an aid in preventing the continuance of this abnormal condition.
The first thing that happens after the line switch has connected the calling subscriber with the first selector is a succession of impulses over the vertical side of the line, this being the set of impulses corresponding in number to the thousands digit or to the office, if there is more than one. It will be understood that here we are considering a single office of ten-thousand-line capacity or thereabouts, and that, therefore, this first set of impulses corresponds to the thousands digit in the called subscriber"s line. Each one of these impulses will flow from the battery through the vertical relay and each movement of this relay armature will close the circuit of the vertical magnet and cause the shaft of the selector to be stepped up to the proper level. Immediately following the first series of selecting impulses from the subscriber"s station, a single impulse follows over the rotary side of the line. This gives the rotary relay armature one impulse and this in turn closes the circuit of the private magnet once.
The single movement of the private magnet armature allows the escapement finger on the arm _5_ to move one step and this brings the side switch contacts into the second position.
_Second Position of Side Switch._ In this position lever _4_ of the side switch places a ground on the wire leading through the rotary magnet to a source of interrupted battery current. The impulses which thus flow through the rotary magnet occur at a frequency dependent upon the battery interrupter and this is at a rate of approximately fifteen impulses per second. The rotary magnet will step the selector shaft rapidly around until something occurs to stop these impulses. This something is the finding by the private wiper of an ungrounded private contact in the bank, since all of the contacts corresponding to busy trunks are grounded, as will be explained.
The action of the private magnet enters into this operation in the following way: A circuit may be traced from the battery through the private magnet to the third side switch wiper when in its second position, thence through the back release relay to the private wiper. If the wiper is at the time on the private bank contact of a busy trunk, it will find that contact grounded and the private magnet will be energized. The energizing of this magnet will not, however, cause the release of the side switch. It must be energized and de-energized. The private magnet armature will, therefore, be operated by the finger of the rotary magnet armature on the first rotary step. The private magnet will be energized and hold its armature operated if the private wiper finds a ground on the first bank contact and will stay energized as long as the private wiper is pa.s.sing over private contacts of busy trunks.
Its armature will not be allowed to fall back during the pa.s.sage of the wiper from one trunk to another, because during that interval the finger of the rotary magnet will hold it operated. As soon, however, as the private wiper reaches the private bank contact of an idle trunk, no ground will be found and the circuit of the private magnet will be left open. When the impulse through the rotary magnet ceases, the private magnet armature will fall back and the side switch will be released to its third position.
_Third Position of Side Switch._ The first thing to be noted in this position is that the calling line is cut straight through to the second selector trunk, the connection being clean with no magnets bridged across or tapped off. The third wiper of the side switch, when in its third position, is grounded and this connects the release wire of the second selector trunk, on which the switch wipers rest, through the private wiper, the winding of the back release magnet, and the third wiper of the side switch to ground. This establishes a path for the subsequent release current through the back release magnet; and, of equal importance, it places a ground on the private bank contact of that trunk so that the private wiper of any other switch will be prevented from stopping on the contacts of this trunk in the same manner that the wiper of this switch was prevented from stopping on other trunks that were already in use.